1. Field of the Invention
This invention relates to a hydraulic apparatus including a piston for putting a load thereon; a hydraulic cylinder for causing the piston to move vertically; a hydraulic fixed delivery pump for supplying an operating oil to the hydraulic cylinder; a normally closed valve connected between the hydraulic pump and the hydraulic cylinder; a check valve connected in parallel to the normally closed valve to permit the flow of the operating oil only toward the hydraulic cylinder; an oil tank; a normally open valve connected to a pipe passage for transmitting to the oil tank the operating oil delivered from the hydraulic pump; a control circuit for piston-raising operation for controlling the normally open valve so as to smaller the oil passage thereof in the case of raising the piston and so as to totally open the normally open valve in the case of lowering the piston; and a control circuit for piston-lowering operation, for controlling the normally closed valve so as to totally close the same in the case of raising the piston and so as to enlarge the oil passage thereof in the case of lowering the piston.
2. Description of the Prior Art
Conventionally, various hydraulic apparatuses of the above-mentioned type were in practical use. A typical one of such apparatuses is a hydraulic elevator apparatus which is constructed as follows. A load is put on the upper end of a piston which is driven vertically through the interior of the hydraulic cylinder. When the piston is raised, an operating oil is supplied from a hydraulic fixed delivery pump to the hydraulic cylinder through an appropriate hydraulic circuit or circuits. When the load is lowered, the operating oil in the cylinder is discharged. The flow of the operating oil delivered from the pump, namely, the flow of the pressure oil, is parted into two, one of which is supplied to an oil tank through a normally open valve and the other of which is supplied to the lower end portion of the cylinder through a parallel circuit which includes a check valve and a normally closed valve. The check valve is disposed with its flow direction so set as to check the flow of the operating oil from the hydraulic cylinder toward the pump.
Even when the pump is being operated, only if the normally closed valve is totally closed and the normally open valve is totally opened, the operating oil delivered from the pump is allowed to flow into the oil tank and is prevented from being supplied to the cylinder. Thus, the piston is neither raised nor lowered.
When it is desired to raise the piston, the opening of the normally open valve is reduced in size. Then, the operating oil flowing through the normally open valve is reduced in volume. The operating oil of the amount which corresponds to this reduction is supplied under pressure into the cylinder by passing through the check valve. Thus, the piston is raised. The more the opening of the normally open valve is reduced in size, the more the piston velocity is increased. If this valve is totally closed, the piston velocity will reach its highest value.
When it is desired to lower the piston, the pump is kept inoperative and, the normally open valve is totally opened and the normally closed valve is appropriately opened. The pressure oil inside the hydraulic cylinder is discharged therefrom owing to the weight of the piston and load and is allowed to pass through each of the normally open and normally closed valves to flow into the oil tank. Thus, the piston is lowered. The more the opening of the normally closed valve is enlarged, the more the piston velocity is increased.
When vertically moving the piston by controlling the normally open valve and normally closed valves, the piston usually is driven as follows. First, the piston is caused to start its movement from its resting position or its first level position toward a desired level position. Then, the piston velocity is increased at a specified acceleration to reach a specified first piston velocity. The position which corresponds to the commencement of this velocity is now referred to as a second level position. Thereafter, the piston is moved at said first velocity to reach a position at which the piston starts to be so decelerated as to smoothly and precisely stop at the desired level position. This position is referred to as a third level position. Such deceleration is effected at a specified deceleration, whereby the piston velocity is decelerated towards a second specified velocity. The position of the piston which corresponds to the commencement of this second velocity is now referred to as a fourth level position. Thereafter, the piston is moved at the second specified velocity to reach a specified level position. This position is referred to as a fifth level position. The piston which has passed over this fifth position is kept moving during the time it is being decelerated substantially at a fixed rate and is stopped at a sixth level position which is the desired level position. The above-mentioned piston velocity, piston acceleration, piston deceleration and piston-velocity change-over position are so chosen as to permit the piston to stop smoothly at its specified position as promptly as it is permitted.
In the process of the piston movement, the piston velocity, piston acceleration or piston deceleration is changed over at the above-mentioned first to sixth level positions. Accordingly a force is applied, to the piston, i.e., an impact force which moves the piston with a large acceleration and large acceleration change with time. This movement is herein called "impact movement". This force is determined by change in the quantity of operating oil flowing into, or out of, the hydraulic cylinder due to opening or closing of the normally open valve or normally closed valve. The larger this change, the greater this impact force. This creates a danger in operating the apparatus or causes damage to the apparatus itself, and when the apparatus is used for an elevator, makes it uncomfortable to ride on.
From the foregoing description as to the opening or closing operation of the normally open valve and normally closed valve at the time of raising or lowering the piston, the following will be understood. At the time of raising the piston, the change-over of the piston velocity at the above-mentioned level positions 1, 4, 5 and 6 is effected when the normally open valve is totally opened or almost so opened. Further, the change-over of the piston velocity at the above-mentioned, level positions 2 and 3 is effected when the normally open valve is totally closed or almost so closed. At the time of lowering the piston, the change-over of the piston velocity at the level positions 1, 4, 5 and 6 is effected when the normally closed valve is totally closed or nearly so closed. Further, the change-over of the piston velocity at the level positions 2 and 3 is effected when the normally closed valve is totally opened or almost so opened.
The usual valve which is used as the normally open valve and the normally closed valve for the conventional apparatus has the characteristics that, in the region of small opening, the rate of change of the valve opening based upon the change in the valve stroke is low, and accordingly the rate of change of the oil quantity passing through the valve based upon the change in the valve stroke is low. In the region of the large valve opening however, the respective rates of changes of the valve opening and oil quantity are high. At the time of raising the piston, therefore, the impact movement caused to the piston at the second and third level positions is small and the impact movement caused thereto at the first, fourth, fifth and sixth level positions is large. At the time of lowering the piston, the former impact movement is large and the latter one is small. In this way, the impact movement which occurs in the piston may vary with the direction in which the piston is moved and the degree of opening given when the piston receives the impact force. Since in this way the impact force acting on the piston varies with the conditions when it is produced, the mechanical strength of the piston-cylinder section and the mechanisms associated therewith must be respectively made strong enough to resist the maximum impact force. This is not preferable in designing the apparatus.
It is also not preferable since in the case of using the apparatus for an elevator the apparatus becomes uncomfortable to ride on since the user is given a feeling of discomfort or uncertainty.